SERIES I ARTICLE Origin(?) of the Universe 6. Present Challenges in Cosmology Jayant V Narlikar The final article in the series reviews the strengths and weaknesses of the big bang cosmology, conceptual as well as observational. It is argued that despite its many successes there are enough question marks against this cosmology to keep the issue open. In particular, an Jayant V Narlikar, Director, alternative view-point developed by the author and his Inter·University Centre colleagues is described. In this the universe has always for Astronomy and been in existence. Known as the Quasi-Steady State Astrophysics, works on action at a distance in Cosmology it combines some good features of the big bang physics, new theories of cosmology with new ideas which may help resolve some of gravitation and new the outstanding questions of today. models of the universe. He has made strong efforts to promote teaching and Strengths and Conceptual Weaknesses of the Big research in astronomy in Bang Cosmology the universities and also writes extensively in English and Marathi for a In this concluding part of the series on cosmology we shall wider audience on science take a critical look at the big bang models and then make and other topics. some projections for the future. To begin with, we consider the strengths of these models. Recall from Part 3 of the series that Einstein's general theory of relativity led Alexander Friedmann to expanding world models in 1922, and seven years later Hubble's law that received a simple interpretation This six-part series will wi thin the Friedmann models was discovered. cover: 1. Historical Back­ ground. 2. The Expanding Then in the 1940s, extrapolation of these models to epochs Universe. 3. The Big very close to the Big Bang led George Gamow and his Sang. 4. The First Three students Ralph Alpher and Robert Hermann to the concept. Minutes. 5. Observational of primordial nucleosynthesis. And from their considerations Cosmology. 6. Present Challenges in Cosmology. of the early universe emerged the idea of a present day background of isotropic radiation. Both these concepts of relic --------J\I\AArv-------- SERIES I ARTICLE nuclei and relic radiation received observational backing in the 1960s and 1970s through the measured abundances of light nuclei (4He, 2H, etc.) and the microwave background (see Part 4 of the series). These are then the plus points of the big bang picture entitling it to a prima-facie position of trust. In any emerging branch of science such a theory is needed at the beginning. But after the early stages are completed and the subject progresses, one must take a more critical look at the theory, to see how far it fits the more detailed experiments / obser­ vations that inevitably follow. When we carry out such an exercise for the standard hot big bang cosmology, several disquieting features begin to show up. Singularity: The concept of big bang origin itself marks a There are many plus departure from standard physics. The event cannot be points of the big described by standard techniques of theoretical physics : all bang picture entitling the equations break down. Mathematicians would call this a it to a prima-facie singular event, i.e., one where no standard mathematical position of trust. But, techniques work. To the physicist, the appearance of all a critical look the matter in the universe at t > 0 with no discussion of reveals several what went on at t = 0, violates its conservation laws. disquieting features. Some big bang cosmologists regard this as the signature of the profundity of the Big Bang. They argue that this event marks the origin of the universe including the origin of science and its laws; as such it lies beyond the scope of science. Other cosmologists regard this as a sign of incompleteness of our present understanding of science: perhaps the singularity would 'go away' when we learn more mature physical laws than we now know (e.g., quantum gravity). Inflation: Approach of the latter kind led in the 1980s to the now well known idea of the inflationary universe. D Kazanas, K Sato and A Guth during 1980-81 independently suggested this idea. It takes into consideration the so-called grand unified SERIES I ARTICLE Figure 1 The Horizon Ef­ fect. The past light cone of o observer 0 at r = 0, ep­ och t, terminates at the big bang epoch t = O. There it has a radial coordinate ex­ tent of rH (say). Particles a like a lying beyond this distance don't causally af­ fect the observer at 0 while particles like b lying within this cone do. The limited value of rH for small t places a severe limit on the extent to which theories (GUTs: see Part 4 of the series) which seek to unify the universe can be homo­ all laws of physics into one single comprehensive framework. geneous. Particle physicists believe that GUTs will be significantly effective for very high energy particles, such as those found shortly after the big bang. Typically, GUT-energies are of the order of 1016 GeV per particle (1 GeV approximately equals the energy store of a proton) and such energies per particle may arise about 10-36 second after the big bang. Just as a temperature of 100 Celsius marks a change of state for water to steam or vice versa, so does this energy mark a phase transition for the universe in the inflationary theory. As the universe expands it 'cools' with the particle energies 15 16 dropping continuously. At _10 _ 10 Gev, the GUTs suggest a change of the lowest energy state of matter, normally designated as 'vacuum' in quantum theory. The universe discovers that extra energy is suddenly available for it to expand very rapidly. This same extra energy of steam condensing to water appears as latent heat. This rapid expansion is called inflation. Just as compound interest grows much faster than simple interest, an inflationary universe expands very rapidly. This rapid ------------------------------~~A~------------------_-p~-~-~-IA-~-.~-P-,--.. -.,.-.,-n-n£ SERIES I ARTICLE Figure 2 The scale factors of some of the different de Sitter C types of models with a non-zero cosmological constant are shown. In a typical model, by choosing Einstein universe A close to the critical value Ac, we can have a universe which expands from 0 to A, stays nearly static over an arbitrarily long stretch from A to B and then ex­ o pands from B to the present C. Note that over expansion lasts a very short time but is enough to make the the stretch Be the Univer­ initial size larger by a factor as high as 1050. sal expansion tends to accelerate. The figure also Rapid expansion of this kind produces some lasting effects in shows the 1917 models of the universe. One is that it helps to make it more Einstein and deSitter. hOInogeneous. Imagine a typical region 10-36 second after the big bang. Any physical effect will travel in it at most with the 10 speed of light, c = 3 X 10 cm/s and so will not cover a distance of more than 3 x 10-26 cm. Normally therefore we would expect regions of this size to be 'homogenized' at 10-36s. Had there been no inflation, the normal slower expansion would have further increased their size at most to - 10 cm to 1 metre at the present epoch. Known as the horizon effect, this had been a ticklish problem for the standard Friedmann models. Why do we find the universe homogeneous on scales of - 1026 metres when it should have been so only on scales of at the most 1 metre? The problem disappears if we assume the inflationary phase in between. The inflationary phase also flattens the universe, that is, it severely diminishes the effect of the curvature term k / S2 in -R~--O-NA--N-C-~-L--III-IV-_ L~-~----------------~~~------------------------------- SERIES I ARTICLE the expansion of the universe (see Parts 3 and 4 of the series). Thus it strongly predicts that the present universe should be almost indistinguishable from the flat (k = 0) case. An added advantage of the inflationary model is the way it allows any initial (primordial) inhomogeneities in the universe to grow. It predicts a spectrum of inhomogeneities which does not depend on scale. The present studies of large scale structure from galaxies to superclusters substantiate this prediction. In spite of these attractive features the inflationary models also have their own conceptual problems. There have been several detailed inflationary models based on speculations in high energy particle physics, but they seem contrived and fine-tuned to get specific results. The now well known idea of the Some Practical Weaknesses of Big Bang Cosmology inflationary universe was born in the Let us now look at a few of the problems that the big bang 1980's to handle the cosmology is currently facing. singularity problem. The age problem: We referred to it in the last part of this series. If we accept the inflationary big bang model and the current estimates of Hubble's constant then the age of the universe is no greater than 8-10 billion years. This is far too short to accommodate galactic and stellar ages in the range of 12-18 billion years. If we assume that all these values are correct, how does the big bang concept survive? Recall that in Part 3 of the series we had referred to the cosmological constant A , first introduced by Einstein, then discarded as unnecessary but still available to the theoretician if needed as an extra parameter.